What's the point of unlocking class meetings within a single complex course?

In the window Specify the complex courses, a small red anchor appears at the top right of the class meetings displayed on the grid.

These anchors indicate that the location of class meetings within the group is set, and will not be changed during automatic placement. Depending on the intended result, you may wish to disable these anchors by un-ticking the box Lock the class meetings at the bottom left of the window.

For example, if you choose the distribution mode The teachers change groups half way through the course, it's not necessarily necessary for teacher A to see group 1 first and then group 2. By un-ticking the box Lock the class meetings, you offer greater flexibility to the software, which will choose the best possible configuration to optimize the timetable.